1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless communication method, a propagation measurement apparatus and a propagation measurement method for receiving a spectrum-spread transmission signal. In particular, the present invention concerns a wireless communication apparatus, a wireless communication method, a propagation measurement apparatus and a propagation measurement method for performing propagation measurement using a preamble section of the spectrum-spread transmission signal.
More specifically, the present invention relates to a wireless communication apparatus, a wireless communication method, a propagation measurement apparatus and a propagation measurement method for performing propagation measurement by despread of each spread code in the baseband section having a clock frequency not more than a chip rate in an RF section. In particular, the present invention concerns a wireless communication apparatus, a wireless communication method, a propagation measurement apparatus and a propagation measurement method for performing propagation measurement using a plurality of despreaders that despread each chip of a short code, in consideration of a trade-off between speed enhancement and circuit scale and power consumption.
2. Description of the Related Art
Canonical standards concerning wireless networks can include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see non-patent document 1), HiperLAN\2 (e.g., see non-patent document 2 or 3), IEEE802.15.3, and Bluetooth communication, for example. The IEEE802.11 has enhanced standards such as IEEE802.11a, IEEE802.11b, etc. depending on differences of wireless communication systems and frequency bands.
Further, in recent years, so-called “ultra-wideband (UWB) communication” for carrying out wireless communication that uses a very wide frequency band such as 3 GHz-10 GHz is attracting attention as a short-distance, ultra-high-speed wireless communication system, and is expected to be put to practical use (e.g., see non-patent document 4). At present, a data transmission system having a packet structure including a preamble is being developed as an access control system of ultra-wideband (UWB) communication, in IEEE802.15.3 and the like.
The UWB communication has high time resolution by employing very narrow pulses, and this property enables “Ranging” for radar and positioning. In particular, recent UWB communications can include high-speed data transmission exceeding 100 Mbps (e.g., see patent document 1) along with the original ranging function (e.g., see patent document 1).
It is anticipated that WPAN (Wireless Personal Access Network) represented by the UWB as short-distance communication will be employed in various kinds of household electrical appliances and CE (Consumer Electronics) devices in the feature, and home networks and P-to-P transmission exceeding 100 Mbps between CE devices are expected to be achieved. If the use of millimeter wave bands becomes widespread, it becomes possible to achieve short-distance wireless communication exceeding 1 Gbps and also an ultra-high-speed DAN (Device Area Network) for short-distance communication including a storage device etc.
A recent trend is to put SS (Spread Spectrum) based wireless LAN systems to practical use. With the spread spectrum, even if there is communication using the same frequency in the neighborhood, a C/I required for enabling normal communication can be set below 0 dB. That is, even if a communication apparatus detects a signal of another apparatus at the same level as that of the apparatus, the apparatus can still communicate. In particular, the spread spectrum is convenient to use in the UWB since the occupied bandwidth of the UWB is originally much wider than the required bit rate.
The SS systems include the DS (Direct Spread) system. According to this system, the transmission side multiplies an information signal by a random code sequence called a PN (Pseudo Noise) code to spread an occupied band for transmission. The reception side multiplies the received spread information signal by the PN code to despread the information signal for reproduction.
The UWB transmission system includes two types: DS-UWB and impulse-UWB. The DS-UWB system maximizes spread speeds of DS information signals. The impulse-UWB system employs an impulse signal sequence having a very short period of approximately several hundreds of picoseconds.
The DS-UWB system can control spectra using PN code speeds, but needs to operate logic circuits with a high speed of the order of GHz, so that undesirably the power consumption easily increases. On the other hand, the impulse-UWB system can be configured in combination with a pulse generator and a low-speed logic circuit, so that there is an advantage of reducing the current consumption. However, disadvantageously, it is difficult to control spectra using the pulse generator.
Both systems can achieve high-speed data transmission by spreading signals to an ultra wide frequency band such as 3 GHz to 10 GHz for transmission and reception. The occupied bandwidth is of the order of GHz, and the occupied bandwidth divided by the center frequency (e.g., 1 GHz-10 GHz) is approximately 1. The occupied bandwidth is ultra wideband compared to bandwidths normally used in wireless LANs based on the W-CDMA or cdma2000 system, and the SS (Spread Spectrum) or OFDM (Orthogonal Frequency Division Multiplexing) system.
Conventionally, there has been used a Gaussian monocycle pulse as an impulse signal for UWB transmission. Let us compare a Gaussian monocycle pulse with a rectangular wave monocycle pulse to examine requirements for the device linearity in pulse generation. An example here uses the rectangular wave monocycle pulse of Tp=200 [ps] and 1 [V]. The Gaussian monocycle pulse follows the equation below. In the equation, constants such as 3.16 and 3.3 are found to provide a spectrum equivalent to the rectangular wave monocycle pulse.
                              x          ⁢                                          ⁢                      (            t            )                          =                  3.16          ⁢                      t                          T              p                                ⁢                      exp            ⁢                                                  [                                          (                                  3.3                  ⁢                                      t                                          T                      p                                                                      )                            2                        ]                                              (        1        )            
FIG. 23 shows time waveforms. FIG. 24 shows a comparison between frequency characteristics of power spectrum densities for these monocycle pulses. The power spectrum densities [W/Hz=J] in the case where the voltage pulses are transmitted at one pulse per second and are driven under the condition of 50 ohms are shown in FIG. 24.
As can be seen from FIG. 24, a pulse of 100 MHz will yield the power density 80 dB higher than this value. The pulse peak here indicates a power density of approximately −211 dBJ. Consequently, the pulse of 100 MHz yields approximately −131.3 [dBW/Hz=dBJ] just equivalent to the FCC specification of −41.3 [dBm/MHz].
Therefore, the following can be concluded.
(1) The Gaussian monocycle pulse is almost the same as the rectangular wave monocycle pulse in the transmission band. (2) The Gaussian monocycle pulse generates a higher peak voltage than the rectangular wave monocycle pulse, requires the linearity, and makes processing difficult including power amplification.
The conventional UWB communication uses monocycle pulses. FIG. 25 shows the frequency characteristics of power spectrum densities in FIG. 24 in terms of antilogarithms instead of decibels. Though there is no special need for using antilogarithms, the linear representation of energy provides many intuitive benefits.
The spectrum has the following two requirements.
(1) The FCC specifications for spectrum masks disable radiation of 3 GHz or less. (2) The band ranging from 4.9 GHz to 5.3 GHz contains a 5-GHz wireless LAN that should be avoided.
The following can be observed from the linearly displayed power spectrum.
(1) If the above-mentioned requirements are not satisfied, only about half of the power [3 dB] is transmitted. (2) A pulse wave form is expected to be disturbed. The receiving side allows just another half of the energy to pass through a matched filter. (3) There is caused a loss of 6 dB or more in total.
FIG. 26 shows a configuration example (conventional example) of a receiver in the UWB communication system. The configuration of the receiver in FIG. 26 is similar to that of a DS-SS (direct sequence spread spectrum) receiver.
The example in FIG. 26 assumes that the VCO oscillates at the same frequency as the pulse frequency.
The receiver follows the VCO timing and generates a pulse train having data all set to zeros. Using this, the receiver generates three waveforms each deviated for half of pulse width Tp, i.e., Tp/2, and multiplies them by a received signal.
By intentionally deviating the VCO frequency a small amount at the time of detecting a pulse position, a pulse timing match will occur at some point in time (Sliding Correlation).
When a pulse timing match occurs, the energy increases as a result of the multiplication, making it possible to detect the pulse position.
When the pulse position is detected, the intentionally deviated VCO frequency is returned to the correct frequency. At the same time, a tracking operation takes place in order to maintain this timing.
The received signal is multiplied by the waveforms deviated for ±Tp/2 against the center (puncture) to find energies. Differences are used to detect positive and negative values corresponding to positive and negative pulse position errors. These values are supplied to loop filters and are used as control voltages for the pulse position tracking.
However, the receiver configured as shown in FIG. 26 needs to divide a signal path into three and use three multiplication-oriented circuits, thus complicating the circuitry.
Further, the receiver needs to change frequencies for search and tracking operations. The time needed for this changeover prolongs the time for synchronization establishment.
Pulse positions need to be correctly detected under a noise environment. For this purpose, it is necessary to detect that the energy increases more than once. A frequency is intentionally deviated by a slight amount. After the energy increases more than once, resulting values are averaged. Thereafter, pulse positions needed to be detected. Consequently, the time to establish the synchronization becomes lengthy.
Analog circuits are used to configure systems for frequency deviation and tracking. However, analog circuits are often complex and are subject to variance, making it difficult to ensure stable operations.
Energy values are used for the pulse position detection and tracking, thus degrading the S/N ratio and characteristics.
[Patent document 1] PCT Japanese Translation Patent Publication No. 2002-517001
[Non-patent document 1] International Standard ISO/IEC 8802-11:1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent document 2] ETSI Standard ETSI TS 101-761-1V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part1: Basic Data Transport Functions
[Non-patent document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (RLC) sublayer
[Non-patent document 4] NIKKEI ELECTRONICS Mar. 11, 2002, pp. 55-66 “Ultra Wideband: Revolutionary Wireless Technology is Born”
In addition to the above-mentioned related art, there have been proposed by the present assignee a technique related to the present invention as disclosed in U.S. patent application Ser. No. 2004-0179582.